Ordered semigroup

In mathematics, an ordered semigroup is a semigroup (S,•) together with a partial order ≤ that is compatible with the semigroup operation, meaning that x ≤ y implies z•x ≤ z•y and x•z ≤ y•z for all x, y, z in S.

If S is a group and it is ordered as a semigroup, one obtains the notion of ordered group, and similarly if S is a monoid it may be called ordered monoid. The terms posemigroup, pogroup and pomonoid are also in use. Additive semigroup of natural numbers (N,+) and additive group of integers (Z,+) endowed with natural order are examples of a posemigroup and pogroup. On the other hand, (N∪{0},+) with the natural order is a pomonoid. Clearly, every semigroup can be treated as a posemigroup endowed with the trivial (discrete) partial order: '='. The class of all semigroups may therefore be viewed as a subclass of the class of all posemigroups (indeed one may then prefer to denote a posemigroup by a triple (S,•,≤)).